A phase-locked loop (PLL) refers to an automatic control closed-loop system including a phase detector, a loop filter, a voltage control oscillator (VCO), and the like. The PLL can complete phase synchronization between two electrical signals, and therefore is widely applied to fields such as broadcast communication, frequency synthesis, automatic control, and clock synchronization. Stability of the PLL is affected by noise and fluctuation of a power source. Therefore, in actual application, a low dropout regulator (LDO) usually needs to be used to overcome impact of the noise and fluctuation of the power source on the PLL, to ensure a feature of a noise-sensitive circuit such as the PLL.
As shown in FIG. 1, a prior-art LDO may include a reference voltage source 101, an error amplifier 102, a compensation circuit 103, a transistor 104, a bleeder circuit 105, and a load 106. The compensation circuit includes a nulling resistor 1031 and a Miller compensation capacitor 1032, and the bleeder circuit includes a first bleeder resistor 1051 and a second bleeder resistor 1052. Due to existence of the compensation circuit, noise distribution of the prior-art LDO is shown in FIG. 1, where Vn,op2 represents equivalent input noise of the error amplifier 102, Vn,bg2 represents output noise of the reference voltage source 101, Vn2,R12 represents thermal noise of the resistor 1051, and Vn,R22 represents thermal noise of the resistor 1052. It can be learned from the noise distribution of the prior-art LDO that, in the prior-art LDO, noise performance of the LDO can be improved by significantly increasing a transconductance of the error amplifier.
However, a minimum value exists in layout implementation of the nulling resistor, and the minimum value is usually of a magnitude of 10 ohms; and a maximum value of a capacitance also exists in layout implementation of the Miller compensation capacitor. Therefore, a gain-bandwidth product of the error amplifier has a maximum value lower limit. A transconductance upper limit of the error amplifier is relatively low because the gain-bandwidth product of the error amplifier has a maximum value lower limit. Once the transconductance of the error amplifier exceeds the upper limit, system stability of the LDO is compromised.
Therefore, it can be learned that, because the transconductance upper limit of the error amplifier is relatively low, noise performance of the prior-art LDO is relatively poor.